1. Field of the Invention
The present invention relates to an optical pickup device forming an optical recording/reading apparatus, such as, what is called an optical disk system, a magneto-optical disk system, an optical card system.
2. Description of the Related Art
Hitherto, optical disks, such as CDs (compact disks) (trademark) and DVDs (digital versatile disks) (trademark), have been proposed as optical recording media. In addition, optical pickup devices for writing information signals to and reading out information signals from such optical disks have been proposed.
In recent years, in order to increase the recording capacity of such optical disks, in optical pickup devices, the wavelengths of light beams emitted from a light source are made shorter, and the NA (numerical aperture) of an objective lens for converging the light beams on a signal recording surface of such optical disks is made high.
For example, the wavelength of light emitted from a semiconductor laser (LD) serving as a light source in an optical pickup device is 780 nm in an optical pickup device for a CD and is 650 nm in an optical pickup device for a DVD having a larger recording capacity. The numerical aperture (NA) of an objective lens in an optical pickup device is 0.45 in an optical pickup device for a CD and is 0.60 in an optical pickup device for a DVD.
A semiconductor laser having a light-emitting wavelength of 405 nm (blue violet) and an objective lens having a numerical aperture (NA) of 0.85 have been proposed. By using such a semiconductor laser and an objective lens in the optical pickup device, the recording capacity of an optical disk is further increased.
However, when, as mentioned above, the light-emitting wavelength of a semiconductor laser (light source) is made shorter, and the numerical aperture of an objective lens is made high, wave aberrations are considerably increased in an optical system due to various production errors. More specifically, in the case where production errors are about the same, the shorter the light-emitting wavelength of a semiconductor laser is made, or the higher the numerical ratio of an objective lens is made, the larger the wave aberrations become. When wave aberrations in an optical system increase, optical performance is reduced, so that proper writing and reading out of information signals cannot be carried out.
Accordingly, there has been proposed, for example, a related optical pickup device for a DVD including a mechanism for changing relative tiltings between the optical pickup device and an optical disk, so that when skewing of the optical disk occurs, the optical axis is always made perpendicular to a read-out surface of the optical disk.
However, it is not possible to properly correct various wave aberrations that occur due to the occurrence of various production errors in an optical device by only adjusting the tilting of the entire optical pickup device with respect to the optical disk in this way. In particular, it is not possible to correct wave aberrations caused by thickness and tilting errors of the optical recording medium.
Various other structures for correcting wave aberrations in an optical pickup device have hitherto been proposed. One example is a structure using a liquid crystal device. In this structure, the liquid crystal device is inserted at a light path between a light source and an objective lens and, by the liquid crystal device, a desired phase distribution is given to the transmission light. In other words, in this structure, a phase that is opposite to that produced by any wave aberrations that occur is previously given to incident light using the liquid crystal device, so that there are no aberrations at an image-forming surface.
In the liquid crystal device, substrates which sandwich liquid crystal molecules are ordinarily planar glass substrates. Electrodes for applying an electrical voltage to the liquid crystals are formed on the substrates. The liquid crystal molecules are lined up along alignment films formed on the glass substrates and are moved by the application of an electrical voltage using the electrodes formed on the substrates. Such movements of the liquid crystal molecules change the refractive index of the entire liquid crystal device, so that the phase of the transmission light that passes through the liquid crystal device can be changed. In order to give a phase distribution to the transmission light, an electrical voltage distribution is created in the electrical voltage applied using the electrodes. An example of a simple structure for achieving this is formed by dividing the electrodes into at least two portions. By separately applying voltages corresponding to desired phase distributions to the divided electrode portions, voltage distributions which match the number of electrodes and the applied voltages are formed, so that nearly desired phase distributions are provided in the transmission light. Obviously, when the number of divisions of the electrodes is larger, so that the divided electrode portions are smaller, it is possible to produce an ideal phase distribution.
However, in such a liquid crystal device, the larger the number of divisions of the electrodes, the larger the area of electrodeless portions formed between the electrode portions becomes. Since the refractive indices of the liquid crystals at the electrodeless portions differ from the refractive indices of the liquid crystals at the electrode portions, not only is image formation adversely affected, but also diffraction of light and other phenomena occur at the boundary between the liquid crystals at the electrode portions, thereby resulting in a loss in the quantity of light. If a semiconductor laser serving as a light source has enough power, such a loss in the quantity of light caused by the diffraction of light is not a problem, but in order to increase the life of the semiconductor laser, it is desirable that the loss in the quantity of light in the liquid crystal device be small.